1. Field of the Invention
This invention relates to a high-strength hot-rolled steel sheet having special advantage for use as inner plates, chassis parts and strength members of motor vehicles, and having a tensile strength of 70 to 100 kgf/mm.sup.2, and further relates to a novel method of manufacturing the steel sheet.
2. Description of the Related Art
Conventionally, high strength steel sheets have widely been used to form inner plates, chassis parts and strength members of motor vehicles in order to reduce the weight of the vehicle body. High strength is required for safety's sake; other properties, e.g., good formability or workability under working, typically, pressing, and good fatigue resistance characteristics after working are also required.
Cold-rolled steel sheets have often been used as steel sheets satisfying these conditions. However, to reduce manufacturing cost, hot-rolled steel sheets have frequently been adopted in recent years.
Further strength improvement of hot-rolled steel sheets is required to enable further reduction of vehicle body weight because of recent strict motor vehicle regulations. Products in a tensile strength (TS) range of 70 to 100 kgf/mm.sup.2 are now increasingly used over those in a TS range of 50 to 60 kgf/mm.sup.2.
With respect to such high-strength hot-rolled steel sheets, typical important properties are:
(1) stable high strength with only small variations of quality and consistency, PA0 (2) low yield ratio, PA0 (3) ease of production requiring no severe hot-rolling conditions, PA0 (4) improved spot welding workability, PA0 (5) improved fatigue properties, and PA0 (6) improved rolled shape.
Many different methods are available for strengthening conventional hot-rolled steel sheets having a tensile strength of 50 to 60 kgf/mm.sup.2. Known examples include solid-solution strengthening, structure strengthening, precipitation strengthening and grain refining strengthening. Such strengthening methods are used to manufacture various items to obtain optimum quality and economical features for each item.
For strengthening hot-rolled steel sheets having a tensile strength in the range of about 70 to 100 kgf/mm.sup.2, however, available strengthening means are very limited. It is a problem that high strength cannot be achieved by treatment that is mainly based on solid-solution strengthening or grain refining strengthening. Even by precipitation strengthening enabling improved weldability and stable manufacturing, it is difficult to achieve a tensile strength higher than 80 kgf/mm.sup.2. In fact, substantially no practical manufacturing means is available, other than structure strengthening with pearlite or bainite, or precipitation strengthening.
Precipitation-strengthened high strength steel has a high yield ratio (ordinarily 0.80 or higher). In particular, with respect to steel having a tensile strength of 80 kgf/mm.sup.2 or larger, the yield ratio is so high that the spring-back of the steel after pressing is excessive for many purposes.
On the other hand, structure-strengthened steel is advantageous in that it entails no considerable incompatibility between strength increase yield reduction. For example, a ferrite-martensite dual phase mixture steel called dual phase steel and disclosed in Japanese Patent Publication Sho 61-15128 has highly improved elongation characteristics and fatigue resistance characteristics. Also with respect to this structure-strengthened steel, if a TS of 80 kgf/mm.sup.2 or larger is required, strict manufacturing conditions must be followed; otherwise, serious shape defects or variations of quality occur in the manufacturing process.
Japanese Patent Laid-Open Hei 1-312032 also discloses a dual phase steel having a ferrite-martensite mixture structure. However, the TS of this steel is very low, such as 50 to 72 kgf/mm.sup.2.
An (.alpha.+.gamma.) structure steel having a tensile strength of 80 to 100 kgf/mm.sup.2, called TRIP steel, is also disclosed in Japanese Patent Laid-Open Hei 3-10049. This TRIP steel is a high-strength steel and achieves its characteristic properties by particularly weighting the workability factor. In the case of this TRIP steel, however, the tensile strength is greatly influenced by phase percentages in the steel, in particular, the amount of retained austenite. For this reason it is very difficult to produce the steel with uniform quality. This is particularly true with respect to quality uniformity along the widthwise and lengthwise directions of the steel band. Moreover, the carbon content of this steel is so high that its spot welding weldability inevitably deteriorates.
For the foregoing and other reasons, there is no steel presently available which satisfies the important requirements for a desirable low-yield-ratio high-strength hot-rolled steel sheet.